Download A STUDY OF GORGONIAN FLABELLUM IN ST. GEORGE`S

 A STUDY OF GORGONIAN FLABELLUM IN ST. GEORGE’S BERMUDA: THE
RELATIONSHIP BETWEEN SIZE, DEPTH, AND WATER INTERACTION
Emily Claire Braun
Department of Biology, Clark University, Worcester, MA 01610
([email protected])
Abstract Gorgonians are found on reefs throughout the world’s oceans. They are
located at a large variety of depths within the water column. The species Gorgonian
flabellum are seen in different sizes throughout reefs. Previous studies have shown that
there is a relationship between the orientation of gorgonians and their size. As the
ocean’s water moves, the gorgonians are pushed back and forth by the water’s force.
This study looks at both depth and width of Gorgonian flabellum and wave interaction,
as reflected in oscillation rates, to determine if there is a correlation between the two.
Key Words: Gorgonian; orientation; oscillation
Introduction
Gorgonian flabellum are commonly known as purple sea fans. These organisms are
formed of polyps. The polyps form a colony that takes on the fan-like shape. Gorgonian
flabellum is a filter feeder that captures small organic matter, such as plankton, in the
organism’s clusters of polyps. The structure of a gorgonian is maintained by a strong,
larger, mother branch, that grows by adding smaller daughter branches (Sanchez et. al.,
2004). This allows gorgonians to become “multi-branched treelike networks” that can
grow up to several meters tall (Sanchez et. al.). This network of branches forms the
“fan-like” shape of the Gorgonian flabellum colonies. The main support branch of the
colony is smaller than the holdfast, which connects the fan to the substratum rock
(Muzik and Wainwright, 1977). This creates a sturdier base than the fan shaped colony
itself, allowing for the colony to oscillate according to water movement, while
maintaining the strong connection of the holdfast to the rock. The sturdier base allows
for the fans to maintain their orientation, even when the roughness of the water
increases (Muzik and Wainwright).
Several studies have researched the link between gorgonian shape and orientation.
These studies have led to the theory that gorgonian fans orient at right angles to the
dominant flow of the ocean’s current. This placement is due to the mechanical forces
caused by the water’s movement (Grigg, 1972). As there is movement throughout the
water channel, the forces act upon the gorgonian fan. Water motion also appears to
have an effect on a fan’s size. In shallower waters, the strength of the wave force that
acts upon the gorgonian seems to be associated with branch diameter (Grigg, 1972).
Size Variation in Gorgonian flabellum Braun The purpose of this study was to determine if there is a correlation between orientation
and size of Gorgonian flabellum. This study specifically examines the depth,
orientation, and size of Gorgonian flabellums found in St. George’s, Bermuda. In
addition to size and depth, I have also tested for a relationship between water
motion/wave interaction and the size of the gorgonian.
Materials and Methods
The study sites that were used were Tobacco Bay and Whalebone Bay, located in St.
George’s Bermuda. Fifty percent of the data were collected at each of the locations.
Whalebone Bay and Tobacco Bay both had areas that received higher levels of wave
interaction, as well as well-protected sections of the reefs. Both orientation types were
selected for this study.
Gorgonian flabellum organisms at Tobacco Bay and Whalebone Bay were randomly
selected to sample. Once located by snorkeling, researchers measured the width of the
fan, with a tape measure, in what appeared to be the widest part. Figure 1
demonstrates how the widest part of the gorgonian was determined and where the
researchers measured for the width. Depth was then measured by using a weight that
was attached to a string with measurements marked along it. One researcher held the
top, as close as possible, directly above fan. The second researcher aligned the bottom
of the measuring string with the weight to where the holdfast of the Gorgonian flabellum
met with the rock. This was repeated for ten measurements at Tobacco Bay and ten
measurements at Whalebone Bay.
Figure 1: Gorgonian flabellum, line represents where the
measurements were made.
To determine if there is a relationship between wave interaction and the width or the
depth of the Gorgonian flabellum the purple sea fans were video recorded for 30seconds. For each Gorgonian videoed, an oscillation tally number, the number of times
that the fan moved in one direction or the other, was recorded. This was done by
Journal of Marine Ecology @ Clark University Volume 1: Issue 1 page 92
Size Variation in Gorgonian flabellum Braun observing the 30-second video, twice; if the oscillation numbers differed between
viewings, then a third viewing was done.
Correlation coefficients were then calculated for all of the data (depth, width, and
oscillation) and tested for significance.
Results and Discussion
Figure 2 shows the positive correlation between depth and width of the measured
gorgonians purple sea fan, which is highly significant (t18=5.12, p<0.001). The overall
trend shows that as the depth of a purple sea fan increases, then the width of the widest
part of the fan also increases. The data support the hypothesis that if a Gorgonian
flabellum is located in deeper waters, then it is likely to be larger than those in shallower
waters.
Figure 2: The width of Gorgonian flabellum compared to depth at Whalebone Bay
and Tobacco Bay, in St. George’s Bermuda, October 10th – 12th 2012.
Figures 3 and 4 depict the oscillation rates of the purple sea fans depending on width
and depth. Figure 3 shows the relationship between the purple sea fan’s number of
times oscillated within 30 seconds and its depth. The slope of the line shows that there
is no significant relationship between number of oscillations and the depth of the
gorgonian. The fan depth did not affect the number of oscillations. The correlation
coefficient is not significant (t8=0.00, p=1.00). Figure 4 shows the relationship between
the Gorgonian flabellum’s number of times oscillated within 30 seconds and its width.
While the slope of the regression suggests some relationship between oscillation and
depth, the relationship is not significant (t8=0.06, p=0.95).
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Size Variation in Gorgonian flabellum Braun Figure 3: Width of Gorgonian flabellum compared to wave interaction.
Whalebone Bay and Tobacco Bay, St. George’s, Bermuda, October 10th – 12th
2012.
Figure 4: Depth of Gorgonian flabellum compared to wave interaction. Whalebone Bay
and Tobacco Bay, St. George’s, Bermuda, October 10th – 12th 2012.
Some options for further research on this topic would to obtain a larger sample. This
could help further investigate the current hypothesis of the apparent positive correlation
between Gorgonian depth and size. A possible change in the study methodology to help
expand the sample size and further evaluate this hypothesis would be to use scuba to
collect data. This will allow for a larger variety of depths of Gorgonians, as well as
facilitate measurements of width and depth. The measurements may have been
Journal of Marine Ecology @ Clark University Volume 1: Issue 1 page 94
Size Variation in Gorgonian flabellum Braun affected by human error. Because of the lung capacity of the researchers, some
measurements may have been rushed while collecting. This issue could be resolved by
using scuba as a means of research, rather than snorkeling. Another change to this
study that would be helpful for further research is to use a computer graphics program
to measure the fans. This would allow for possibly more accurate data of the
measurements.
Acknowledgements
I thank my fellow Clark University researchers who travelled to Bermuda, specifically
Sarah Carpenter, for accompanying me on the snorkeling exhibition and for all of her
help in the water. Also, thanks to my parents, Beth Essig and Jeff Braun, for funding my
Bermuda field study and being extremely supportive. Lastly, I thank my professors, Deb
Robertson and Todd Livdahl for all their knowledge, help, and patience.
Literature Cited
Grigg, R. W. "Orientation and Growth Form of Sea Fans." Limnology and
Oceanography 17.2 (197): 185-92. Web.
Kolla, V. "A Review of Sinuous Channel Avulsion Patterns in Some Major Deep-sea
Fans and Factors Controlling Them." Marine and Petroleum Geology 24.6-9
(2007): 450-69. Web. 23 Oct. 2012.
Muzik, K., and S. Wainwright. "Morphology and Habitat of Five Fijian Sea Fans."
Bulletin of Marine Science 27.2 (1977): 308-37. Web.
Sanchez, Juan A., H. R. Lasker, E. G. Nepomuceno, J. D. Sanchez, and M. J.
Woldenberg. "Branching and Self-Organization in Marine Modular Colonial
Organisms: A Model." The American Naturalist 163.3 (2004): E24-39.
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